1. Field of the Invention
The present invention relates to a mirror body and an optical device using the mirror body.
2. Description of the Related Art
The wavelengths currently used for optical systems include radiowave, infrared ray, visible light, ultra violet light and X-ray. The ultra high frequency wave is referred to as cm wave, mm wave and microwave, depending on the wavelength concerned. The wavelength of the infrared light is 1 mm to 760 nm, and the wavelength of the visible light is 760 to 380 nm. No definite definitions are found for ultraviolet light and X-ray; however, usually, the light in the wavelength range from 380 to 1 nm is referred to as the ultraviolet light and the wavelengths from a few 10 nm to 0.001 nm correspond to X-ray.
For the purpose of resource exploration and meteorological observation, the wavelength regions of microwave and visible, near infrared and infrared light are used. Communication between the satellites and the ground mainly relies on the wavelengths in the GHz band used in the transponder and the optical communication in the wavelengths from 800 nm to 1,000 nm. In these years, the use of the semiconductor laser device around 400 nm, shorter than the conventionally used wavelengths, for the communication between satellites is being studied, because such shorter wavelengths lead to smaller loss due to the air. Additionally, for the purpose of sampling scientific information, observations in the ultraviolet to X-ray region (for example, X-ray astronomy, etc.) have come to be actively carried out. The optical systems handling these wide ranges of wavelengths are required to have a high degree of smoothness with respect to the mirrors. The degree of smoothness of the surface is determined on the basis of the shorter wavelengths of the used wavelengths.
When a mirror is used for handling image information, the degree of smoothness thereof affects the resolution, while when used for communication, the low degree of smoothness thereof results in power loss. Consequently, the degree of smoothness of λ/20 to λ/50 is required for communication, which is higher than the degree of smoothness of λ/10 required for handling image information. Additionally, miniaturization of semiconductors causes reflecting mirrors for the X-ray region to be also used for X-tray lithography, and even for the X-ray having a wavelength of 13 nm, a degree of smoothness of the order of 0.8 nm is required.
In particular, the optical systems for use in space are used in space indeed, and the means for putting the systems on the orbits are spacecrafts such as rockets, so that the systems are required not only to be made high in precision but also to be made light in weight.
Low thermal expansion glass is extremely small in linear expansion coefficient, and has been adopted for the mirror substrates for large optical systems on the ground. Owing to this fact, many large and high-precision optical systems have adopted low thermal expansion glass for the mirror substrate material.
However, low thermal expansion glass is low in rigidity and strength as a material, and hence it is not suitable for reduction of weight and additionally, for the uses requiring the degree of smoothness of the order of 1 nm.
Beryllium is a metal and suitable for weight reduction as far as the rigidity and strength of the material are concerned. On the other hand, beryllium is a metallic material and large in thermal expansion coefficient, but its linear expansion coefficient is also large, so that beryllium is not suitable for high precision mirrors for use in space. Additionally, beryllium is harmful and is restricted in the processing place therefor, and moreover is associated with a problem such that a workpiece of beryllium should be processed after its surface is coated with another metal such as nickel.
Sintered compact is light in weight, and is high in both rigidity and strength, thus having the properties suitable as substrates of mirrors for use in space. For example, Japanese Patent Laid-Open No. 9-178919 discloses an example in which aluminum nitride sintered compact is used as substrates of mirrors for use in optical systems. Aluminum nitride sintered compact is suitable as far as it is light in weight, rigid and strong, but it is low in thermal conductivity, and moreover abundant in pores, so that it is not suitable for mirror substrates required to have smooth surface.
The adhesive strength between sintered compact and a metal to form the reflecting surface is weak, so that, as disclosed in Japanese Patent Laid-Open 9-178919, it is necessary that a gold thin film to be the reflecting film should be formed on a sintered compact to be used as a mirror substrate through the intermediary of a glass containing Al2O3/glass layer.
For silicon carbide, research and development thereof has been promoted as materials, high in rigidity and strength, for use in high precision and lightweight optical systems.
Silicon carbide is smaller in linear thermal expansion coefficient than beryllium, but is high in rigidity and strength; silicon carbide is larger in linear thermal expansion coefficient than low thermal expansion glass, but is high in thermal conductivity and the temperature distribution hardly tends to be nonuniform; thus silicon carbide is suitable for substrates of mirrors in high precision optical systems for use in space.
However, there has been a problem that sintered silicon carbide, carbon fiber reinforced silicon carbide, and chemical vapor phase grown silicon carbide are not suitable for substrates of mirrors used in large optical systems.
Sintered silicon carbide is available in two different forms: one is a high purity sintered silicon carbide obtained by sintering a high purity silicon carbide powder at temperatures 2,000° C. or higher, and the other is a composite ceramic sintered compact, as disclosed in Japanese Patent Laid-Open No. 1-188454, in which silicon carbide particles are dispersed in a polycrystalline (Al2O3) matrix made of anisotropic particles.
The production of silicon carbide on the basis of the chemical vapor phase growth method uses the crystal growth, caused by chemical reaction, from the vapor phase at a high temperature, permitting attaining nearly ideal properties of silicon carbide. Additionally, the above described growth method permits obtaining dense crystals, so that silicon carbide crystals can be grown on the structure bodies made of materials on which high purity sintered silicon carbide and carbon fiber reinforced silicon carbide can be deposited by means of the chemical vapor phase growth method.
Silicon carbide obtained by the chemical vapor phase growth can yield, when mirror finish polishing is made, a mirror which is free from the generation of pores and 1 nm or less in concavities and convexities, and accordingly an ideal mirror available at present as a mirror for an optical system. However, the degree of difficulty of the growth method concerned is increased with increasing mirror substrate size, in relation to the apparatus for chemical vapor phase growth and the growth control of silicon carbide, so that at present an optical system involving an aperture of 0.6 m or more can hardly be produced.
Additionally, a substrate is needed for chemical vapor phase growth, and when sintered silicon carbide is adopted for the substrate, the shape precision offers problems as will be described later on. When a material other than silicon carbide is used for the substrate, the silicon carbide obtained by the chemical vapor phase growth and the substrate are different from each other in thermal expansion coefficient, so that there occur problems such that cracks are generated in the silicon carbide film and the exfoliation of the metal layer forming the reflecting surface from the substrate occurs when silicon carbide is deposited at the time of production, and under such a large temperature variation condition as occurring in space.
A method may be conceivable in which the substrate is removed after the growth of the silicon carbide film; however, this case is also accompanied by the problems associated with production such that cracks are generated in the silicon carbide film when silicon carbide is deposited, and additionally, the film growth is required to yield a larger thickness.
On the contrary, sintered silicon carbide undergoes a contraction as large as 20% in sintering. This makes it difficult to maintain the shape precision. Additionally, the produced silicon carbide is porous, and hence pores of the order of 2 μm are found to occupy about 2% of the surface area when the surface is smoothed. This is the reason why the surface smoothing is required to be performed by depositing silicon carbide by means of the chemical vapor phase growth. The constraint involving deposition by the chemical vapor phase growth also imposes an additional constraint on the upsizing of mirrors.
Carbon fiber reinforced silicon carbide is manufactured in such a way that carbon fiber reinforced graphite is formed by burning a substrate made of a carbon fiber reinforced plastic, and at 1,400° C., silicon is impregnated into the graphite thus obtained and is made to react with the graphite. This production method is suitable for upsizing because this method uses carbon fiber reinforced plastic as the substrate. However, this method makes the carbon fiber to be partially involved in the reaction to form silicon carbide, so that the silicon carbide thus obtained cannot fully enjoy the properties such that the Young's modulus thereof is smaller and is higher in rigidity compared to sintered silicon carbide and silicon carbide produced by the chemical vapor phase growth method. Additionally, the surface of the silicon carbide produced by this method does not permit attaining a degree of smoothness suitable for optical systems for use in the visible light region even when polished because carbon fibers are contained therein. Consequently, the silicon carbide concerned can be used merely for the optical systems for use in the middle and far infrared regions for which the wavelengths are long. For the purpose of using for the optical systems in the visible light region, it is necessary to carry out the silicon carbide coating by means of the chemical vapor phase growth, so that the silicon carbide concerned is also not suitable for production of large size optical mirrors because of the constraint imposed by the chemical vapor phase growth method similarly to the case of the high purity sintered silicon carbide.
It is essential that a mirror substrate either for use in space and for use on the ground is made to be lightweight.
However, for the purpose of being used in optical systems, the mirror substrate is required to be high in rigidity and strength, and moreover, small in linear expansion coefficient and high in thermal conductivity, and yet moreover, high in machining precision.
It is necessary that sintered materials are produced by sintering at high temperatures, and contraction as large as about 20% takes place. Consequently, the shape precision can hardly be maintained.
Additionally, as for a sintered compact as a mirror substrate, for example, in the case of an aluminum nitride sintered compact disclosed in Japanese Patent Laid-Open No. 9-178919, the degree of smoothness of the surface is low, and moreover, the adhesion strength to metal is low, and accordingly it is necessary that a glass containing Al2O3 layer is formed, and additionally a glass layer is formed on the glass containing Al2O3 layer, and a reflecting film is formed on the glass layer; thus aluminum nitride sintered compact is not suitable for the mirror substrate for use in a high precision optical system.
On the other hand, in the case of a composite ceramic sintered compact in which silicon carbide particles are dispersed in a polycrystalline (Al2O3) matrix made of anisotropic particles, there is a problem that when mirror finish polished, pores are found on the surface.
Although silicon carbide is a material suitable for use in space as far as the rigidity, strength and thermal conductivity are concerned, high purity sintered silicon carbide, carbon fiber reinforced silicon carbide and chemical vapor phase growth silicon carbide all do not meet all the requirements, and are not suitable for upsizing.